[Support] Ensure redirected outputs don't contain output from previous tests.

stdout may be buffered, and may not flush on every write. Explicitly flushing
before redirecting the output ensures that the captured output does not contain
output from other tests.

llvm-svn: 360617

llvm/unittests/Support/CommandLineTest.cpp

@@ -1085,6 +1085,7 @@ public:
   // Return std::string because the output of a failing EXPECT check is
   // unreadable for StringRef. It also avoids any lifetime issues.
   template <typename... Ts> std::string runTest(Ts... OptionAttributes) {
+    outs().flush();  // flush any output from previous tests
     AutoDeleteFile File;
     {
       OutputRedirector Stdout(fileno(stdout));